Werk #11687: mk_postgres supports CentOS7

Component	Agent bakery
Title	mk_postgres supports CentOS7
Date	Nov 12, 2020
Level	Trivial Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	2.0.0b1 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME)

added native ensure_str implementation

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